The most common method of producing primary aluminium from its ores is the so-called Hall-Heroult process. This involves dissolving aluminium ore (containing Al2O3) in molten cryolite (Na3AlF6). AlF3 is also usually present in the mixture to reduce the melting point of cryolite. The mixture is electrolysed, which mobilizes the aluminium ions in a liquid phase. In the presence of carbon, Al2O3 is reduced to elemental aluminium, and the carbon is oxidised to carbon monoxide. The electrolysis of the aluminium oxide is carried out in “pots”, the internal walls and bottom of which are formed from carbon blocks, which are typically joined with a conductive material. These pots form part of the cathode during the electrolysis. The carbon linings of the pot are typically surrounded externally by refractory firebricks and insulating bricks, which usually contain silica and/or alumina. Over a period of years of continual use, the carbon of the pots will absorb salts from the molten ore/cryolite mixture, resulting in their deterioration, at which point the pots needs to be replaced. When SPL is removed, it is prepared and separated into a “first cut” and a “second cut”. The first cut refers to the carbonaceous material from the cathode lining, while the second cut comprises mostly refractory material. The waste or ‘spent’ pot liner (SPL) material typically contains one or more of carbon, silica, alumina, aluminium, sodium salts, aluminium salts, fluoride salts, cyanides and traces of heavy metals. Because of the reactive and harmful nature of these species, the SPL material needs to be handled and disposed of carefully to avoid danger to human health and to the environment. This is becoming increasingly important in view of environmental legislation being brought into force in many countries.
A number of treatments of SPL materials have been suggested in the prior art, none of which is entirely satisfactory.
There are two general approaches for the treatment of SPL waste: 1) hydrometallurgical treatment and 2) thermal treatment. Around the world there are only a few purpose built plants that treat SPL, which indicates the problems faced in producing a safe and commercially viable method of treating SPL material.
Hydrometallurgical Treatments
An example of a hydrometallurgical treatment of SPL is the Low Caustic Leaching and Liming process (LCLL Process) developed by Alcan. It is a three step process that requires the use of complicated reactors.
In a first step, finely ground SPL material is leached in a caustic solution to remove the fluorine, free and complexed cyanide, alumina, and some silica into the leach liquor at around 85° C. In a second step, more sodium hydroxide is added at elevated pressure and temperature to destroy the cyanide in the leach solution while producing sodium fluoride. In a final step, more caustic material (generally lime) is added to the fluoride liquor to produce calcium fluoride and a recyclable, caustic leach solution. This process requires significant capital expenditure for the processing equipment and is only commercially viable on a large scale (80,000 tonnes/year). In addition, it is claimed to generate more waste by mass as a by-product than it treats.
Thermal Treatments
Several technologies for the thermal treatment of SPL have been investigated, some of which are discussed below.
Efforts have been made to use SPL as a fuel source for rockwool manufacture or by co-firing in cement kiln. Both processes can be problematic due to the impact of SPL on the final product and more importantly due to permitting and regulatory issues for co-firing a hazardous waste product. It is only deemed commercially viable when SPL material is available at large scale and not suitable as a proximal, smaller scale solution.
Alcoa have investigated the Top Submerged Lance process developed by Ausmelt for the treatment of SPL. This process is disclosed in the International patent publication no. WO94/22604. In this process, the SPL material is smelted with a submerged lance in a furnace at temperatures of 1150° C. to 1250° C. while an oxygen-containing gas is injected directly into the SPL material. The temperature is sufficiently high to destroy all cyanides and organic materials. The energy to sustain operations at these temperatures is primarily provided by the combustion of the carbon in the SPL. While efficient combustion of the SPL carbon has been demonstrated, this technology produces an off-gas stream which contains high levels of the toxic gases HF and NaF. In order to be commercially viable, the technology needs access to a fluoride plant for HF utilisation for the production of AlF3 that can be recycled back to the primary process, i.e co-location with a primary aluminium plant is required.
Others have investigated the treatment of SPL in a rotary kiln such as described in patents: U.S. Pat. No. 5,711,018, U.S. Pat. No. 5,164,174 and U.S. Pat. No. 4,735,784. While good combustion of the SPL carbon was achieved, the slag shows poor leaching performance and the off-gas contains high levels of fluoride compounds. In addition, the output mass of the processed waste is significantly higher than the input mass of SPL material. Because the process does not produce a useful product or a conditioned waste which is significantly cheaper to dispose of, the economic justification for the capital and operational cost of implementing such procedures for the treatment of SPL is problematic.
Elkem Technology have investigated the treatment of SPL in an electrode arc furnace. Crushed SPL is supplied to a closed electrothermic furnace together with a SiO2 source as a glass forming flux material and Fe2O3 as oxidation agent. Fe2O3 is reduced by the SPL carbon to produce CO/CO2 and metallic iron which forms a separate phase from the slag. A source of CaO is used to react with all fluoride present to form CaF2. This process is described in U.S. Pat. No. 5,286,274. While this process is efficient in trapping the fluorine as CaF2 in the slag, the amount of oxidant agent required for the complete combustion of the SPL carbon is higher than the amount of treated SPL. Having an oxidising agent and graphite electrodes submerged in the slag melt pool will result in a high consumption of the electrodes. In addition, the process is only commercially viable if the reduced Fe2O3 can be recovered as metallic iron. Thus, the plant has to be designed accordingly which results in a significant increase in capital costs.
Columbia Ventures Corporation describes the treatment of SPL in a plasma torch furnace in International patent publication no. WO 93/21479. SPL material is fed into a plasma furnace with water or steam as an oxidant and exposed to the heat of a plasma torch. The SPL carbon is converted to CO or CO2 and the fluoride is driven off as HF, which then needs to be further treated, since it cannot be released into the environment due to its harmful nature. The plasma torches described in this document are water-cooled and those exemplified are typically made from metallic components. The present inventors have found that in the harsh chemical and thermal conditions of the reactor containing high temperature airborne fluorine species the torches quickly corrode, limiting the commercial viability of the process. Further, it is described that the torch is of the transferred type, with the anode being centered coaxially within the tube and the cathode being the materials undergoing treatment or the container surface itself. In the example, the container is graphite, i.e. electrically conducting. The typical composition of SPL is such, that it is only electrically conductive in its liquid state, thus an external heat source would have to be used to provide a melt pool during start up of the process. The present inventors have found that when the container surface (or crucible surface) is electrically conductive and used as the cathode, control of the arc tends to be very difficult. It would be desirable to develop a method that does not require the pre-heating of the SPL material and allows more control over the arc during the process.
More stringent regulations prohibit the landfill disposal of untreated SPL and the competent authorities generally refuse to compromise the environmental standards in view of the possible legal challenges they may face. However, in some cases, derogations for landfilling are granted and will continue to be in place unless an alternative solution appears. The UK Environmental Agency (EA) and the US Environmental Protection Agency (EPA) cannot be commercially biased and they can only select technologies that are industrially available, therefore; the solution must be available, scaled and technically superior (Best Available Technique (BAT) in the UK, Best Demonstrated Available Technology (BDAT) in the US) to be a mandatory requirement. This gives rise to a position where the primary aluminium industry is in need of technological development for treatment technologies, to underpin their primary aluminium production operation. At present, the EA/EPA are not satisfied with the status of industrial solutions and they therefore insist on hazardous landfill destination requirement for all the products resulting from current SPL treatment processes.